Compensatory eye movements in the presence of conflicting canal and otolith signals

Summary Orbital motion of the head with the face directed towards the axis of rotation is a stimulus to the otolith organs which is in the opposite rightwards-leftwards sense to the rotational stimulus to the semicircular canals. This can be experienced, for example, by a child held at arms length en face and swung from side to side. As one swings, say to the right, the child's head rotates to its right yet moves linearly to its left. Eye movement responses to a transient orbital movement were observed whilst subjects fixated earth-fixed targets, i) a near target placed between the head and the axis whose relative displacement is in the same direction as head rotation, and ii) a far target placed beyond the axis whose relative motion is in the opposite direction to head rotation. The motion stimuli evoked slow phase eye movements at 45 ms latency, always in the opposite direction to head rotation, thus compensating for the motion of the far target but in the wrong direction for fixating the near target. Theoretically, fixating the near target demands a predominance of the otolith ocularreflex, which would give an eye movement in the correct direction. However, despite visual cues, it seems that if the canal and otolith-ocular reflexes are evoked in opposing directions, the otolith reflex fails to operate at a sensitivity sufficiently high to reverse the direction of the canal-reflex.     

人工晶体植入前后视觉诱发电位的临床分析

目的:(1)探讨白内障囊外摘除术前术后及人工晶体植入术后视觉诱发电位幅值及潜时变化特征;(2)探讨白内障囊外摘除术术前P—VEP,F—VEP,F—ERG联合预测的必要性。方法:采用重庆泰克医电仪器公司产TEC—100C视觉电生理仪。对白内障患者实行分组测量。术前均测F—ERG,P—VEP,部分病例增测F—VEP。结果:114例术前F—ERG正常,P—VEP测量其P_(100)波幅值潜时均异常:P_(100)波幅值降低,潜时延长,术后及人工晶体植入后幅值,潜时都有所改善,而并发性白内障,代谢性白内障变化不明显。结论:据视觉诱发电位的变化,老年性白内障组,外伤性白内障组术后应及时装入人工晶体,并发性白内障组,代谢性白内障组装入人工晶体临床意义不大。术前联合测试(VEP ERG)对视力预后、以及眼底病变的定量诊断意义重大。     

Eye Movements and Visually Active Dreams

There are a number of reports suggesting an association between profusion of eye movements and active dreaming. It has however been suggested that this relationship might only be evident in comparisons across the night and would not be evident in comparisons within one REM period. Data from 20 subjects taking placebo, amylobarbitone, and nitrazepam were used to test this. Dream reports were collected from REM awakenings and rated blind as visually active or passive. Eye movement profusion (number of 2 sec epochs) was assessed for each REM period. Correlation between dream content and eye movement was low but significant in comparisons including the whole night, and including data from drug, withdrawal, and placebo conditions. A significant correlation was not consistently obtained, however, when data from each REMP were considered separately. Correlations based on data from non-drug nights only were also small and could have been due to chance effects alone. The low correlations were not explicable solely by poor reliability of content ratings. It is concluded that the relationship between visually active dreaming and eye movement is slight, and may not hold when time of night is adequately controlled.     

Eye position signals in human saccadic processing

Summary 1. We studied saccades to briefly flashed targets in 8 human subjects. The target flash occurred (i) during smooth pursuit (ramp-flash), (ii) just before a saccade to another target (step-flash), or (iii) during steady fixation (flash only). All lights were extinguished after the target flash so that smooth pursuit or saccadic eye movements occurred during the interval of complete darkness between the target flash and the saccade to it. We compared these saccades to those made without intervening eye movement (flash only), and quantified the extent to which the saccadic system compensated for the change in eye position that occurred during the dark interval. 2. Saccades to control flashes were reasonably accurate (mean gain 0.87) and consistent. Compensation for the intervening eye movement in the ramp-flash and step-flash paradigms was highly variable from trial to trial. On average, subjects compensated for 27% of the intervening pursuit eye movement on ramp-flash trials and for 58% of intervening saccadic movement on step-flash trials. 3. Multiple regression analysis showed that the variability did not depend on factors such as variations in underlying saccadic gain, response latency, timing of stimuli or size of the required response. We conclude that this variability is intrinsic to saccadic responses that require the use of an eye position signal. 4. These results show that an eye position signal is available to the saccadic system but that this signal has low fidelity. The high variability and low fidelity of the eye position signal suggest that the saccadic system does not normally operate in spatial coordinates, which require the use of an accurate eye position signal, but rather in retinal coordinates.     

Directional organization of eye movement and visual signals in the floccular lobe of the monkey cerebellum

The floccular lobe of the monkey is critical for the generation of visually-guided smooth eye movements. The present experiments reveal physiological correlates of the directional organization in the primate floccular lobe by examining the selectivity for direction of eye motion and visual stimulation in the firing of individual Purkinje cells (PCs) and mossy fibers. During tracking of sinusoidal target motion along different axes in the frontoparallel plane, PCs fell into two classes based on the axis that caused the largest modulation of simple-spike firing rate. For horizontal PCs, the response was maximal during horizontal eye movements, with increases in firing rate during pursuit toward the side of recording (ipsiversive). For vertical PCs, the response was maximal during eye movement along an axis just off pure vertical, with increases in firing rate during pursuit directed downward and slightly contraversive. During pursuit of target motion at constant velocity, PCs again fell into horizontal and vertical classes that matched the results from sinusoidal tracking. In addition, the directional tuning of the sustained eye velocity and transient visual components of the neural responses obtained during constant velocity tracking were very similar. PCs displayed very broad tuning approximating a cosine tuning curve; the mean half-maximum bandwidth of their tuning curves was 170–180 °. Other cerebellar elements, related purely to eye movement and presumed to be mossy fibers, exhibited tuning approximately 40 ° narrower than PCs and had best directions that clustered around the four cardinal directions. Our data indicate that the motion signals encoded by PCs in the monkey floccular lobe are segregated into channels that are consistent with a coordinate system defined by the vestibular apparatus and eye muscles. The differences between the tuning properties exhibited by PCs compared with mossy fibers indicate that a spatial transformation occurs within the floccular lobe.     

The effect of gravity on the horizontal and vertical vestibulo-ocular reflex in the rat

Horizontal and vertical eye movements were recorded in alert pigmented rats using chronically implanted scleral search coils or temporary glue-on coils to test the dependence of the vestibulo-ocular reflex (VOR) upon rotation axis and body orientation. The contributions of semicircular-canal versus otolith-organ signals to the VOR were investigated by providing canal-only (vertical axis) and canal plus otolith (horizontal axis) stimulation conditions. Rotations that stimulated canals only (upright yaw and nose-up roll) produced an accurate VOR during middle- and high-frequency rotations (0.2-2 Hz). However, at frequencies below 0.2 Hz, the canal-only rotations elicited a phase-advanced VOR. The addition of a changing gravity stimulus, and thus dynamic otolith stimulation, to the canal signal (nose-up yaw, on-side yaw, and upright roll) produced a VOR response with accurate phase down to the lowest frequency tested (0.02 Hz). In order to further test the dependence of the VOR on gravitational signals, we tested vertical VOR with the head in an inverted posture (inverted roll). The VOR in this condition was advanced in phase across all frequencies tested. At low frequencies, the VOR during inverted roll was anticompensatory, characterized by slow-phase eye movement in the same direction as head movement. The substantial differences between canalonly VOR and canal plus otolith VOR suggest an important role of otolith organs in rat VOR. Anticompensatory VOR during inverted roll suggests that part of the otolith contribution arises from static tilt signals that are inverted when the head is inverted.     

Acute effects of light and darkness on sleep in the pigeon (Columba livia)

In addition to entraining circadian rhythms, light has acute effects on sleep and wakefulness in mammals. To determine whether light and darkness have similar effects in birds, the only non-mammalian group that displays sleep patterns comparable to mammals, we examined the effects of lighting changes on sleep and wakefulness in the pigeon. We quantified sleep behavior (i.e., bilateral or unilateral eye closure) in pigeons maintained under a 12:12 LD cycle, and immediately following a change from a 12:12 to a 3:3 LD cycle. During both LD cycles, sleep was most prevalent during dark periods. During the 3:3 LD cycle, darkness had the greatest sleep promoting effect during the hours corresponding to the subjective night of the preceding 12:12 LD cycle, whereas light suppressed sleep across circadian phases. As previously suggested, the light-induced decrease in sleep in the subjective night might be partly mediated by the suppression of melatonin by light. Although the sleep promoting effect of darkness was modulated by the circadian rhythm, sleep in darkness occurred during all circadian phases, suggesting that darkness per se may play a direct role in inducing sleep. In addition to the effects of lighting on behavioral state, we observed an overall bias toward more right eye closure under all lighting conditions, possibly reflecting a response to the novel testing environment.     

Canal-otolith interactions driving vertical and horizontal eye movements in the squirrel monkey

The vestibulo-ocular reflex (VOR) was studied in three squirrel monkeys subjected to rotations with the head either centered over, or displaced eccentrically from, the axis of rotation. This was done for several different head orientations relative to gravity in order to determine how canal-mediated angular (aVOR) and otolithmediated linear (lVOR) components of the VOR are combined to generate eye movement responses in three-dimensional space. The aVOR was stimulated in isolation by rotating the head about the axis of rotation in the upright (UP), right-side down (RD), or nose-up (NU) orientations. Horizontal and vertical aVOR responses were compensatory for head rotation over the frequency range 0.25–4.0 Hz, with mean gains near 0.9. The horizontal aVOR was relatively constant across the frequency range, while vertical aVOR gains increased with increasing stimulation frequency. In the NU orientation, compensatory torsional aVOR responses were of relatively low gain (0.54) compared with horizontal and vertical responses, and gains remained constant over the frequency range. When the head was displaced eccentrically, rotation provided the same angular stimuli but added linear stimulus components, due to the centripetal and tangential accelerations acting on the head. By manipulating the orientation of the head relative to gravity and relative to the axis of rotation, the lVOR response could be combined with, or isolated from, the aVOR response. Eccentric rotation in the UP and RD orientations generated aVOR and lVOR responses which acted in the same head plane. Horizontal aVOR-lVOR interactions were recorded when the head was in the UP orientation and facing toward (nose-in) or away from (nose-out) the rotation axis. Similarly, vertical responses were recorded with the head RD and in the nose-out or nose-in positions. For both horizontal and vertical responses, gains were dependent on both the frequency of stimulation and the directions and relative amplitudes of the angular and linear motion components. When subjects were positioned nose-out, the angular and linear stimuli produced synergistic interactions, with the lVOR driving the eyes in the same direction as the aVOR. Gains increased with increasing frequency, consistent with an addition of broad-band aVOR and high-pass lVOR components. When subjects were nose-in, angular and linear stimuli generated eye movements in opposing directions, and gains declined with increasing frequency, consistent with a subtraction of the lVOR from the aVOR. This response pattern was identical for horizontal and vertical eye movements. aVOR and lVOR interactions were also assessed when the two components acted in orthogonal response planes. By rotating the monkeys into the NU orientation, the aVOR acted primarily in the roll plane, generating torsional ocular responses, while the translational (lVOR) component generated horizontal or vertical ocular responses, depending on whether the head was oriented such that linear accelerations acted along the interaural or dorsoventral axes, respectively. Horizontal and vertical lVOR responses were negligible at 0.25 Hz and increased dramatically with increasing frequency. Comparison of the combined responses (UP and RD orientations) with the isolated aVOR (head-centered) and lVOR (NU orientation) responses, indicates that these VOR components sum in a linear fashion during complex head motion.     

Fast, anticipatory smooth-pursuit eye movements appear to depend on a short-term store

Anticipatory smooth pursuit before the expected appearance of a moving target can reduce the initial retinal blur caused by the 100-ms delay of visual feedback. Humans, though, can only voluntarily generate smooth velocities up to about 5°/s without a moving target. However, previous experiments have shown that repetitive brief presentations of a moving target every few seconds appear to charge an internal store, the contents of which can later be released to generate higher velocity anticipatory movements. This store’s longevity was assessed here by repetitively presenting a moving target for 500 ms at different known intervals up to 7.2 s. Target motion at 25°/s or 50°/s was tested, with presentations in alternate directions or the same direction. Anticipatory velocity, measured 100 ms after target onset, decreased with increasing interval for all target motion conditions. A decrease was still seen when accurate timing cues were given before each presentation, suggesting that the drive for anticipatory pursuit is held in a short-term store lasting a few seconds which can enhance the low velocities produced by volition alone. The results also demonstrate that high-velocity anticipatory pursuit helps to overcome the temporal delays in the system and allows target velocity to be matched at an earlier time. Received: 27 August 1997 / Accepted: 22 December 1997